The Food and Drug Administration's Antiviral Drugs Advisory Committee's positive review of Johnson & Johnson (NYSE:JNJ) and Medivir's hepatitis C therapy simeprevir foreshadows the drug's likely FDA approval tomorrow.. Fast on its heels will be Gilead Sciences (NASDAQ:GILD) with its own hepatitis C drug sofosbuvir on Friday. Both drugs have benefited from priority review status.

Hepatitis C is a viral infection of the liver and has multiple genotypes, with genotype 1 the most prevalent. Complications include liver cancer and cirrhosis. Standard of care currently is interferon, which is delivered as a cumbersome injections 2-3 times a week, with only an efficacy of around 50%, and uncomfortable flu-like side effects.

While both Gilead and Johnson & Johnson's compounds are oral pills, they are currently only studied in combination with an interferon (peginterferon-alpha), manufactured by both Merck (NYSE:MRK) as Pegintron and Roche (NASDAQOTH:RHHBY) as Pegasys, and ribavirin, another anti-viral agent. Johnson & Johnson's simeprevir is seeking approval for treatment of genotype 1, while Gilead's sofosbuvir will target genotype 1, 2, 3, 4, 5 and 6.

While the FDA's internal review is a positive step for Johnson & Johnson to gain approval tomorrow, simeprevir is a niche enough drug that competitors won't be squeezed out of the market. Forty-eight percent of hepatitis C patients with genotype 1 had a variant that did not respond to simeprevir versus the control group.

Two other hepatitis C drugs already exist as first generation protease inhibitors: Victrelis (boceprevir) by Merck and Incivek (telaprevir) by Vertex Pharmaceuticals (NASDAQ:VRTX). Both were approved in 2011 and are also used in conjunction with peginterfereon-alpha and ribavirin. The approval of simeprevir could replace these earlier generation protease inhibitors.

Meanwhile, Gilead's sofosbuvir is perhaps seen as the more likely blockbuster, due to efficacy against all genotypes of hepatitis C and against even the most difficult to treat cases of hepatitis C. Even without interferon, sofosbuvir showed 70% cure rates and may be the closest to a truly interferon-free therapy yet.

The hepatitis C market is currently estimated at around $20 billion with potential to expand to $100 billion this decade . The more broad-reaching sofosbuvir expects peak sales at $3.8 billion  and simprevir is estimated annual sales of $400 million.

The race for an oral, non-interferon agent could revolutionize patient care in terms of both reducing patient burden and increasing clinical outcomes. Both Johnson & Johnson with Medivir and Gilead are expected to gain approval this week; neither have experienced significant setbacks or raised concerns that would red-flag their approvals this week. However,  both simeprevir and sofosbuvir are such tiny agents in the vast arsenal of products by each company that FDA approval may or may not be reflected directly in stock price.

Nonetheless, more than 20 other agents are in development to take a piece of the pie for novel hepatitis C agents. AbbVie and Bristol-Myers Squibb are expecting to file New Drug Applications next year. Studies combining the different companies' agents have also yielded impressive cure rates -- over 90% for the combination of J&J's simprevir and Gilead's sofosbuvir, and 100% cure for the combination of Gilead's sofosbuvir and Bristol-Myers Squibb's daclatasvir.

Fool contributor Amy Ho has no position in any stocks mentioned. The Motley Fool recommends Gilead Sciences, Johnson & Johnson, and Vertex Pharmaceuticals. The Motley Fool owns shares of Johnson & Johnson. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.